1. Field
Aspects relate to an image forming apparatus, a system on chip (SoC) unit, and a driving method thereof, and more particularly, to an image forming apparatus and a SoC unit which have a universal serial bus (USB) communication function, and a driving method thereof.
2. Description of the Related Art
A power saving mode refers to a mode in which power supplied to most of the modules is interrupted or minimized in order to minimize power consumption when a system is idle. In order to reduce standby power, a method of interrupting power supplied to a main memory (an external dynamic random access memory (DRAM) in general) or getting a system in a self refresh state and then driving a program in an unused internal memory of a SoC (an internal static random access memory (SRAM) in general) may be used. In general, the SRAM uses a low capacity memory of about 128 KB.
However, if it is necessary to transmit a great amount of data as in a USB driver for a printer or a mass storage device or if the number of supported interfaces increases, a space occupied by stack increases and thus it is difficult to perform all functions of the USB driver in the low capacity SRAM.
Also, since the SRAM is expensive, a high capacity SRAM is ineffective in view of a cost.
If an apparatus used as a USB device interrupts power supplied to the external DRAM in order to enter the power saving mode, the USB driver operating in the external DRAM is also stopped so that the USB device does not communicate with a host device. In particular, in the case of a printer which receives a specific command from a host apparatus through a USB, the printer is not able to perform its original function (a problem that a personal computer recognizes the printer as being disconnected). Therefore, in the related art, the external DRAM is normally operated even in the power saving mode.
However, if power continues to be supplied to the external DRAM, standby power is consumed. On the other hand, if power supplied to the DRAM is interrupted in order to reduce power consumption, the USB driver operating in response to the external DRAM is also stopped so that communication using the USB is impossible. However, in this case, if a program for checking an apparatus status using the USB gets access to the apparatus, it determines that power is turned off and thus there is an inconvenience in using a service. For example, if a personal computer is connected to a printer through a USB and a print monitoring program is installed in the PC, there is a problem that the PC may recognize the printer as being in an abnormal state or being disconnected when the DRAM of the printer is turned off and the USB driver does not operate.